Productivity of Three Young Mixed-Species Plantations Containing N₂-Fixing 'Acacia' and Non-N₂-Fixing 'Eucalyptus' and 'Pinus' Trees in Southeastern Australia

Mixed species plantations have the potential to exceed the biomass production of monocultures. This study examined the productivity of three mixed species plantations in southeastern Australia. Two of these trials contained a 'Eucalyptus' sp. ('E. saligna' Smith or 'E. nitens' [Deane & Maiden] Maiden) planted with 'Acacia mearnsii' De Wild., and the other contained 'Pinus radiata' D. Don with 'A. mearnsii', 'A. decurrens' Willd., 'E. benthamii' Maiden & Cambage, or 'E. smithii' R. Baker. Each trial contained both monocultures and mixtures, and was replicated three or four times. Tree diameters or heights were smaller in mixture than monocultures for some species (P. radiata diameters of 5.9 cm and 7.0 cm in 2:1 mixtures with 'A. mearnsii' and monocultures, respectively) but tended to increase (not significantly) for other species ('E. nitens' diameters of 10.6 cm and 8.5 cm and 'A. mearnsii' diameters of 9.2 cm and 8.8 cm in 1:1 mixtures and monocultures, respectively). As a result, mixtures were intermediate in aboveground biomass production between monocultures of the mixed species in each trial, or they were not significantly different from the monocultures. Competition for resources other than nitrogen (N), such as light, soil moisture, or other nutrients, appeared to balance any positive effects that might have occurred, such as through increased N availability. For example, foliar N concentrations of 'E. saligna' were higher in mixture (23.1 mg g⁻¹) than monoculture (17.7 mg g⁻¹); however, this did not result in greater aboveground tree biomass. The range of different growth responses from mixing different species in this study and in other studies shows that a fundamental understanding of the underlying processes is required to enable a greater predictive capacity of the circumstances under which mixtures can be successful.